1. Field of the Invention
The present invention relates to a process for welding aluminum and aluminum alloys. More particularly, the invention relates to a process for welding the same in a horizontal welding position by applying the weld in a narrow weld groove opened sidewise in the automatic welding of aluminum structural materials having considerable thickness.
2. Description of the Prior Art
Recently, an increasing demand for aluminum and aluminum alloys (which will be referred to simply as aluminum alloys, hereinafter) as structural materials for use in aircraft, vehicles, ships, pressure vessels, and the like has developed. This need stems from the excellent corrosion resistance and high strength of aluminum and aluminum alloys as well as their light weight. As a result, the aluminum materials satisfy industrial demand. For instance, aluminum alloys have been increasingly used as cryogenics for transporting and storage reservoirs containing liquefied natural gases (LNG). Among the various types of reservoirs employed are spherical reservoirs which have thicknesses of 40 to 100 mm, which thicken to 200 mm at the equator. In the welding of this type of reservoir, welding in all positions such as flat positions, the vertical position, the horizontal position and variations thereof must be done.
It has been commonly believed that welding in a horizontal position is particularly difficult for the automatic welding of structural materials of large thickness. This is because of the unique characteristics of welding in a horizontal position, i.e., the influence of gravity on molten metal, which imposes limitations on the amount of deposited metal per any given welding pass. Further, if the amount of deposited metal exceeds the limitations on the amount of metal, welding defects result such as overlaps and the like, so that the amounts of welding current and deposited metal in any given pass should be limited to some extent.
There are many difficulties in the welding of aluminum alloys for the fabrication of structural bodies because of the physical and metallurgical characteristics of aluminum alloys, compared to other structural materials, such as iron-based structural materials.
The following problems are encountered in the welding of aluminum alloys of large thickness:
(i) Steels have melting points of about 1500.degree. C., while aluminum alloys have melting points of about 600.degree. C. Thus, at the same heat input level, aluminum alloys, if an MIG welding process is used, yield greater amounts of deposited metals, which in turn hinder the direct impingement of a welding arc upon the base metal. Consequently, satisfactory penetration is not achieved. In order to cope with this problem, the electric current is increased. However, this results in a further increased amount of deposited metal. Consequently, an undesirable cycle exists in which, as the electric current is increased, the amount of deposited metal is increased. In addition, because the surface of the base aluminum alloy is normally covered with an oxide film when the base alloy is exposed to air, the melting point of the metal surface is about 2020.degree. C. which is much higher than the melting point of the underlying aluminum alloy base. It follows, then, that the mere contact of molten aluminum with the surface of the base metal does not melt the base metal because of the heat stored in the molten aluminum. This is well supported by the fact that when the cross-section of the weld bead is observed, deeper penetration is obtained in the direction in which the electrode is held, while poor penetration results in directions other than that which the electrode is held.
(ii) The thermal conductivity of aluminum alloys is much greater than that of steels, so that the heat applied to the base aluminum alloys not only locally melts a welding zone, but also is dissipated or diffused in only a very short time because of its high thermal conductivity. Consequently, the amount of heat which is used to melt the base metal is limited to some extent. In addition, the heat applied to the base metal which melts the same, as well, is dissipated in only a short time, so that the molten pool solidifies rapidly.
(iii) The welds of aluminum alloys suffer from microfissures which are a unique characteristic thereof. The microfissures referred to herein are minute or hair-line cracks which develop in the deposited metal or portion of the base metal, when the metal is subjected to the repeated cycle of melting, solidification and re-melting. Needless to say, the microfissures exert an adverse effect on the strength of the joint and consequently, it is essential to eliminate such microfissures. In this respect, the method by which the electrode is woven may be one of the causes for the microfissures, and thus the weaving of the electrode should maintain the molten pool to a given size, so that the range of weaving has to be controlled which limits the efficiency of the welding operation.
(iv) Regarding the susceptibility to gas shielding conditions, steels give weld beads of good appearance even if somewhat poor shielding conditions prevail with the result that pits and blow holes form therein, whereas aluminum alloys give weld beads of impaired appearance, even if welding is done under slightly poor shielding conditions. For this reason, close care should be taken while welding, and a high level of skill is required therefor.
Many studies have been undertaken to solve the difficulties and shortcomings encountered in aluminum welding, while taking advantage of the excellent characteristics of aluminum alloys as well as the characteristics of welding in a horizontal welding position. However, no successful methods of welding aluminum alloys in a horizontal position have been reported. The MIG welding process is known as the only acceptable method which gives the highest efficiency in the welding of aluminum alloys in the horizontal welding position. In the MIG welding process, a `V`-shaped weld groove is adopted which has a groove angle of 70.degree. to 90.degree., while the welding electrode is moved in a straight direction along the groove without oscillation. When the edges of base metals are welded, which have a thickness of 50 mm, and which are prepared to form V`-shaped welding grooves which give an X`-shaped weld groove when the base edges are placed together, as many as 35 to 55 welding passes are required on both sides of the weld to secure the base metals which results in time-consuming welding procedures. On the other hand, in the fabrication of spherically-shaped structures which have thicknesses of 100 mm to 200 mm, several hundred passes are required to prepare the welding bead. This not only requires a great amount of manpower, but also it is difficult to obtain welds free of defects.
In order to overcome the shortcomings experienced with the prior art welding processes, it is imperative that the amount of deposited metal per pass be increased. Consequently, a weaving or side-to-side motion of the electrode is employed to increase the amount of deposited metal. However, the weaving motion of the electrode determines the solution of the problems which arise from the characteristics of aluminum alloys. Japanese patent publication Nos. S45-9857 and S47-50504 show welding techniques which use a weaving motion of the tip of the welding electrode in the widthwise direction of the base metals. In this method, an electrode-supplying head portion located outside the welding groove is oscillated, or the rate at which the electrode is fed to the welding groove is varied which causes an oscillating motion of the tip of the electrode in the widthwise direction of the base metals. The methods disclosed are associated with the welding of steels in the vertical welding position. In these processes, only limited success is obtained in achieving good weld zones in the welding of aluminum alloys, in contrast to the welding of steels. Thus, when aluminum is welded according to the conventional welding processes, variations in the welding conditions because of variations in the lengths of the electrode which is projected into the welding groove, exerts a substantial influence on the characteristics of the weld zones, thus causing welding defects such as poor penetration. In addition, when base metals having a thickness as large as 100 mm are to be welded, it is impossible to insert a consumable electrode into the weld groove because the electrodes are not straight and for other reasons.
A need, therefore, continues to exist for a method of welding aluminum and aluminum alloys in which welding defects such as microfissures and poor penetration are eliminated.